1. Field of the Invention
The invention is related to the field of optical communication networks, and in particular, to systems and methods for performing Polarization Mode Dispersion (PMD) measurements on fiber spans that are in-service.
2. Statement of the Problem
Many communication companies use an optical network for transmitting data because of its high-bandwidth capacity. Fiber optic cables in the optical network reliably transport optical signals over long distances between a transmitter and a receiver. The fiber optic cables are comprised of transmission fiber, such as a single mode fiber (SMF). Over the length of SMF, the optical signals experience some degradation due to attenuation along the fiber. Fiber background loss in the fiber causes the attenuation, typically about 0.2 dB/km. The optical signals also degrade due to other limitations, such as Polarization Mode Dispersion (PMD), chromatic dispersion, and nonlinear effects.
PMD is a dynamic pulse broadening phenomena. In a single mode fiber, optical pulses propagating down the fiber may separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses are determined by the state of polarization of the input pulse relative to the fiber's input principal states of polarization (PSP). The separation into the two orthogonal modes may be caused by intrinsic and extrinsic factors. The intrinsic factors may result from fiber manufacturing processes, such as core ellipticity, or built-in asymmetric stresses. The extrinsic factors may be caused by stresses due to twisting, bending, and environmental effects, such as temperature and thermal gradients.
If the core of the fiber has a perfectly circular cross-section, then both modes travel at the same speed over the same distance. Otherwise, one mode travels slower than the other resulting in a difference in group velocities (an effect called birefringence). The difference in velocities between polarization modes is wavelength dependent and time dependent. The difference in propagation time, ατ, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). When the DGD in a fiber becomes excessively large, a receiver is unable to distinguish between a zero bit and a one bit, and bit errors occur eventually resulting in a PMD-induced outage.
When fiber spans are installed, PMD tests are sometimes performed on the fiber spans while the fiber spans are still dark (i.e., no data traffic). The fiber spans are tested one at a time. To test a fiber span, a light source is connected to one end of the fiber span and a PMD measurement system is connected to the other end of the fiber span. The light source is a broadband light source having a bandwidth of about 100 nm. The broadband light source then transmits light over the fiber span, in the 100 nm bandwidth, for receipt by the PMD measurement system. The PMD measurement system then records PMD measurements for the transmitted light.
A broadband light source is used to shorten the time needed for PMD tests. There is an inverse relationship between bandwidth and the test time needed to obtain accurate PMD measurements. As the bandwidth of the light source narrows, the test time increases. As the bandwidth of the light source widens, the test time decreases. For instance, for a 100 nm bandwidth light source, the test time may be five minutes. For a 30 nm bandwidth light source, the test time may be two hours. For a 1 nm bandwidth light source, the test time may be ninety days. Thus, current testing is done with a broadband light source to shorten test time.
One problem with currently using a broadband light source is that testing can only be performed on a dark fiber span. Testing with a broadband light source cannot be performed on a fiber span carrying data signals, as the bandwidth of the broadband light source overlaps the data bandwidth for transporting data signals. For instance, if the data bandwidth for a fiber span in service is the C-band (1530 nm to 1565 nm), and a broadband light source having a bandwidth between 1500 nm and 1600 nm is used, the bandwidth of the broadband light source would overlap with the data bandwidth to corrupt the data. If a fiber span is in-service, the network engineers would have to take the fiber span out of service to perform PMD tests, which is undesirable.
Restricting PMD testing to dark fibers presents problems for network engineers as they may need to perform PMD tests on fiber spans that are in service. For instance, some optical networks are installed to transport data at a data rate of 2.5 Gbps. PMD does not significantly affect 2.5 Gbps networks, so PMD tests probably were not performed on fiber spans in these networks before they were put in-service. If network engineers want to increase a 2.5 Gbps network to a 10 Gbps network or higher, such as a 40 Gbps, then PMD does become an issue at these higher data rates. Network engineers do not know how the networks will perform at the 10 Gbps or higher data rate as PMD tests were not performed on the fiber spans. In order to increase the data rate to 10 Gbps or higher, network engineers would have to perform PMD tests on the fiber spans or risk PMD-induced outages. As stated before, PMD tests would require taking the fiber spans out of service.
A problem remains to provide for efficient in-service PMD testing of fiber spans.